Mobile communications products, such as cellular phone and handsets, are required to be small and light. Such products require radio frequency (RF) filters approximately covering the range 0.5 GHz to 10 GHz to protect the received signal from interference, from the transmitter in the same handset and/or from unwanted externally generated signals. These filters must have low pass-band insertion loss (typically, less than 2 dB) in order to achieve adequate signal-to-noise ratio. Due to their high quality factor, superior power handling capability, low cost packaging on silicon and potential for integration above IC, thin film bulk acoustic wave (BAW) resonators and filters have been widely used in mobile radio communication devices. As shown in FIG. 7, the simplest implementation of a BAW resonator 10 comprises a thin layer 14 of a piezoelectric material, for example, aluminum nitride (AlN), zinc oxide, and PZT, arranged between a bottom metal electrode 13 and a top metal electrode 15. Typically, the BAW resonator 10 is acoustically isolated from a supporting substrate 11 by an acoustic isolator 12, which may include an air cavity formed under a membrane supporting the BAW resonator 10 or an acoustic mirror that includes of a stack of layers alternately formed of high and low acoustic impedance materials. For such a BAW resonator, several wave modes can propagate horizontally along the layers. The modes are formed by such combinations of longitudinal and shear bulk waves traveling at different angles in the layers that can fulfill continuity conditions across layer interfaces and boundary conditions at the extreme ends of the stack. The propagating plate modes are generally called the Lamb waves. A BAW resonator with finite lateral dimensions usually exhibits a multitude of spurious resonances between fs and fp, as shown in FIG. 8.
Performance of such a thin film BAW resonator can be represented by the effective electromechanical coupling coefficient (Kt,eff2) and the quality (Q) factor. The greater the electromechanical coupling coefficient Kt,eff2 becomes, the wider the bandwidth of an RF filter or the tuning range of a voltage controlled resonator can be made.
U.S. Pat. No. 7,280,007 to Feng et al. discloses a technique to increase the quality factor Qp through adding a mass load layer to the resonator perimeter. Although the quality factor Qp is improved by this method, the added mass load layer causes an increase of the shunt capacitance (Co) and reductions of both of Kt,eff2 and Q near series resonant frequency (Qs), which are observed with increasing frame width. This is not ideal in some applications where both Qs and Kt,eff2 should be maximized, for example, UMTS band 1 duplexer. Additionally, the spurious resonances below fs are enhanced by the raised frame. The spurious resonances cause a risk of generating strong ripples in the pass band of a filter.
Therefore, it is desirable to have a resonator structure with an enhanced Qp without compromising Kt,eff2, spurious modes and Qs. Hence, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.